THE CHANNEL ENTRY PROBLEM FOR A YIELD STRESS FLUID

It is first argued that to permit any flow development of the type con sidered, the usual Bingham model must be relaxed to permit some deform ation of the unyielded material. This is done here by adopting a two-v iscosity model in which the unyielded material is modelled as a Newton ian liquid of large viscosity. The problem is then solved in the case of small viscosity ratio and large yield stress. In this way the const itutive model is close to the Bingham model, and the yielded zones occ upy thin boundary layers next to the channel walls. The resulting prob lem is non-linear but the main difficulties concerning the location of the yield surfaces are removed.